The invention relates to a resistor for refractory shaped bodies and to a process for producing the resistor and to shaped bodies derived therefrom.
In the text which follows, the term resistor denotes the provider of the refractory quality and therefore usually also the main component of a refractory shaped body or refractory compounds. In the most general situation, this resistor may be a metal-oxide, mineral, refractory substance, such as MgO, Al2O3, doloma or the like.
In the text which follows, the term elasticizer denotes minerals which, on account of an inherent, relatively high refractory quality but a thermal expansion which differs from that of the resistor, through the formation of microcracks and further effects, lead to an increase in the thermal shock resistance of a mixture of resistor and elasticizer compared to the pure resistor.
Refractory shaped bodies, in particular basic, refractory materials based on magnesia and doloma are used in all high-temperature processes with basic slag attack, for example in the production of cement, lime, dolomite, iron and steel and in the production of non-ferrous metals and in the glass industry, as lining material for kilns, furnaces and vessels. However, if they have a high refractory quality and good chemicals resistance, these materials or shaped bodies are highly brittle, i.e. have a high modulus of elasticity.
In this context, it should be noted that shaped bodies based on fused magnesia are considerably more brittle than shaped bodies based on sintered magnesia. However, by its very nature fused magnesia has a considerably higher resistance to thermochemical attack than sintered magnesia. To this extent, it would be desirable to use fused magnesia or shaped bodies or compounds based on fused magnesia in areas in which there is a high thermochemical attack, in particular an attack from low-viscosity, basic slags. This is the case in particular in rotary tubular kilns for cement production. Particularly in cement rotary tubular kilns, however, there is a considerable mechanical load on the refractory lining, so that known shaped bodies based on-fused magnesia cannot successfully be used in firing units of this type, since their brittleness means that they are unable to withstand the stresses introduced, in particular the ring strains.
Shaped bodies based on fused magnesia are inferior to shaped bodies based on sintered magnesia in firing units of this type even if the shaped body is elasticized.
In the past, numerous measures have been taken for elasticizing, i.e. for improving the thermal shock resistance (TSR) of basic, refractory materials. For example, it is proposed in Harders/Kienow, Feuerfestkunde, Herstellung, Eigenschaften und Verwendung feuerfester Baustoffe [Refractory engineering, production, properties and use of refractory construction materials], Springer-Verlag 1960, Chapter 5.5, page 755, to mix basic, refractory materials with chrome ore, this reference in particular defining the quantity of chrome ore and the optimum grain size fraction of the chrome ore. To achieve a sufficient thermal shock resistance, quantities of chrome ore of between 15 and 30% by weight are required. The elasticizing action, i.e. the action of reducing the modulus of elasticity, of the chrome ore as thermal shock resistance component is explained by W. Spxc3xa4th in xe2x80x9cZur Temperaturwechselbestxc3xa4ndigkeit feuerfester Stoffexe2x80x9d [On the thermal shock resistance of refractory materials], Radex-Rundschau, Volumes 1960-1961, page 673-688, xc3x96sterreichisch-Amerikanische Magnesitaktiengesellschaft, Radenthein/Kxc3xa4rnten, caused by micro-structural stresses on account of different coefficients of thermal expansion between magnesia and chrome ore. However, major drawbacks of the use of chrome ore as a means of improving the thermal shock resistance are that, when the furnace atmosphere is changed, material fatigue occurs and that, as a result of oxidation under the action of alkalis, the chromium oxide which is present in trivalent form in the chrome ore is converted into toxic hexavalent chromium oxide, with all the associated problems with regard to safety at work and disposal.
Furthermore, it is known from Austrian Patent AT 158 208 to add alumina powder, corundum powder and aluminum powder to magnesia bricks in order to improve the thermal shock resistance, spinel being formed in situ during firing of the bricks. The aluminum-magnesium spinel formed is concentrated in the matrix and is in some cases not fully reacted, so that when such bricks are attacked by slags, the matrix, which is of crucial importance for the strength, is preferentially destroyed. Also, a magnesium-aluminum spinel of this type has a different coefficient of thermal expansion from that of pure magnesia, so that in this way micro-structural stresses and therefore microcracks likewise result.
For the first time, it was possible to considerably improve both the thermal shock resistance and the chemicals resistance of magnesia bricks by adding pre-synthesized magnesium-aluminum spinel, in the form of sintered or fused spinel, the quantities added usually being between 15 and 25% by weight. By this measure, it is possible to reduce the modulus of elasticity to approximately 20 kN/mm2. However, as before there are drawbacks in that this spinel component reacts readily with slags, and therefore wear takes place in the region of the spinel matrix, which ultimately also leads to accelerated breakdown of the resistor.
DE 35 27 789 A1 has disclosed a coarse ceramic shaped body which has a microcrack system which is distributed substantially homogeneously in the shaped-body micro-structure. This publication is based on the discovery that a low modulus of elasticity combined, at the same time, with a high resistance to attack from slags can be produced by a microcrack-forming agent of much greater diameter than in the case of, for example, dense oxide-ceramic high-temperature materials being distributed homogeneously in the shaped-body micro-structure, the mechanism being based either on this agent triggering expansion of the relevant particles, i.e. a volume-increasing reaction, during the firing process or the sintering firing of bricks, in which case the microcrack system is then formed in the adjacent, other particles, or considerable shrinkage in the meal fraction being produced, which in turn leads to the described microcrack system in the other particles of the mix. For this purpose, pure magnesia and alumina are mixed in a stoichiometric ratio which corresponds to the magnesium aluminate spinel and are shaped into mixed particles, which are then added to the base batch of sintered magnesia. Refractory shaped bodies of this type have inherently proven their worth. Particularly when used in units which are highly mechanically stressed with a high level of basic slag attack, such as rotary tubular kilns used in the cement industry, however, rapid wear takes place with shaped bodies of this type as well.
DE 44 03 869 A1 has disclosed a refractory, ceramic compound and its use, this refractory, ceramic compound apparently comprising 50 to 97% by weight of sintered MgO and 3 to 50% by weight of a spinel of the hercynite type. In this document, it is stated that, for example for lining industrial furnaces or kilns in which there is a significant mechanical load on the refractory lining, there is a need for products whose brittleness is as low as possible. These furnaces and kilns apparently include rotary kilns used in the cement industry, where kiln deformation can lead to considerable mechanical stresses on the refractory lining, but would also encompass furnaces used in the steel making and nonferrous metals industry, where in particular thermal stresses during heating and in the event of temperature changes would lead to problems. With respect to chrome ore-containing bricks, it is proposed for the elasticizer used to be hercynite or a spinel which is similar to hercynite, in which case the hercynite-like spinel is to lie within the following ranges for the ternary phase diagram FeOxe2x80x94Al2O3xe2x80x94MgO:
23 to 55% by weight of FeO
 less than 15% by weight of MgO
54 to 65% by weight of Al2O3 
 less than 3% by weight of impurities
The refractory bricks which are produced using this spinel and are fired are supposed to have a considerably improved ductility. Furthermore, it is stated in this document that sintered magnesia could also be replaced by fused magnesia. However, with refractory products of this type, it is a drawback that the spinels of the hercynite type tend to dissolve and incorporate MgO from the refractory resistor. The solubility limit of the spinel of the hercynite type for MgO is 15 to 20%. Conversely, MgO is able to take up parts of the hercynite spinel or of the oxides which form it, namely FeO and Al2O3. Therefore, in refractory shaped bodies of this type, undesirable microstructural weakening through diffusion processes and as a result of the constituents partially dissolving one another has been observed, there being a considerable diffusion gradient in the direction from MgO to the spinel on account of the higher diffusion rate of the Mg2+ (W. Schulle, Feuerfeste Werkstoffe [Refractory Materials], Deutscher Verlag fxc3xcr Grundstoffindustrie, 1990, p. 254). A further drawback is that, when using fused magnesia, the elasticizing fraction of the spinel is not sufficiently high to make this resistor suitable for rotary tubular kilns given a sufficiently high thermochemical stability.
It is an object of the invention to provide a refractory resistor which, while having a very high thermochemical stability, also has sufficient ductility even for use in mechanically highly loaded units.
The object is achieved by a refractory resistor having the features set forth below.
A further object is to provide a process for producing the resistor and a refractory shaped body derive therefrom.
This object is achieved by a process having the features set forth below. Advantageous configurations and refinements are characterized below.
According to the invention, a fused magnesia or, in general, a melted and therefore inherently brittle resistor is conditioned in such a manner that it has a high ductility or increased elasticity, so that this resistor, while having a high thermochemical stability, can also be used in mechanically highly loaded units, such as rotary tubular kilns. According to the invention, this is achieved by the fact that spinel-forming agents are added to the resistor during the melting operation, in such a manner that the resistor grains which are formed after the melting have precipitations or precipitation areas in which spinel is concentrated. In this case, the addition of spinel-forming agents is metered in such a manner that the solubility of the resistor for this spinel-forming agent is exceeded, it being possible for the number and, surprisingly, also the size of the precipitation areas to be determined by the precise metering of the spinel-forming agent.
A further surprise is that sufficient elasticizing of the melted resistor is possible, even though no visible microcrack system is thereby generated between the individual fused-magnesia or fused-resistor grains which are joined to one another during the sintering firing.
A further advantage in a resistor which has been conditioned in this manner is that the elasticizer, which in terms of chemicals resistance represents the weak point of the brick, is protected from the slags by the chemically more resistant resistor. In this case, with the inventive added quantities of spinel-forming agents, it is even possible to achieve elasticizing capacities which are so high that the resistor is conditioned to such an extent that, in addition to its xe2x80x9cinternalxe2x80x9d elasticizer (precipitation areas), it does not require any further external additions of elasticizer in the batch.
Naturally, it is nevertheless possible to add further elasticizers (external elasticizers).
It has been found that it is particularly advantageous for the external elasticizer used to be a spinel, as is also employed as an internal elasticizer. This is attributed to the fact that the resistor is so well saturated with the spinel on account of its inherent elasticizing that as a result diffusion is inhibited or reduced. As a result, the influences of diffusion, for example of a pure hercynite, on the resistor are suppressed.
In the principal application area according to the invention, namely fused magnesia, it is preferable to add FeO and Al2O3 or Fe2O3 and Al2O3, so that the precipitation areas are formed substantially from a pleonaste spinel or a spinel of the pleonaste type. It has been found that this elasticizer of the pleonaste type, both internally and externally, has a considerably improved compatibility with the resistor, with sufficient elasticizing being ensured. Furthermore, compared to known elasticizers, this elasticizer has an increased thermochemical resistance.